[Eljaro]

I finally think I found what was wrong. I modified a distributor cap making a big square hole in front of one of the terminal points for cylinder #1 and checked with the timing light
The spark would jump while the rotor point was past the #1 terminal. I did loosen up the screws holding the magnetic pickup and pushed it as far as I could to the most advanced position.
Now the point and terminal match perfectly at idle and at max engine speed.
Apparently MSD does not do a perfect job in phasing the rotor as they claim.

Now the timing is consistent and does not jump around above 3500 rpm.
I also made myself a larger stop bushing (10mm diameter) in order to be able to set 21º initial advance with a max advance of 36º.
The black stop bushing MSD provides did give me with a 21º initial a 42º maximum advance. Some wear of the interior diameter and a not very tight fixing was giving me 3º more advance than the 18º advertised.
I am using the two heavy silver springs.
Runs very well now.